This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from applications for LINEARIZING DEVICE AND METHOD FOR A RADIO COMMUNICATION SYSTEM earlier filed in the Korean Industrial Property Office on the 19th of June 1997, and there duly assigned Ser. No. 25705/1997, RADIO COMMUNICATION SYSTEM EMPLOYING A PREDISTORTION METHOD earlier filed in the Korean Industrial Property Office on the 20th of June 1997, and there duly assigned Ser. No. 26137/1997, and APPARATUS AND METHOD FOR LINEARIZING POWER AMPLIFIER USING PREDISTORTION IN DIGITAL COMMUNICATION SYSTEM earlier filed in the Korean Industrial Property Office on the 10th of October 1997, and there duly assigned Serial No. 52132/1997 copies of which applications are annexed hereto.
1. Technical Field
The present invention relates to a digital predistortion technique for linearizing a power amplifier in a digital communication system, and in particular, to a communication system using a digital predistortion technique to compensate for distortion of an output spectrum caused by non-linear characteristics of a power amplifier.
2. Related Art
In a radio communication system, a transmitter for transmitting an analog signal or digital data is composed of a large number of active elements including a power amplifier. These active elements are designed to increase power efficiency and spectrum efficiency of the radio communication system. However, modulation scheme such as quadrature phase-shift key (QPSK) and quadrature amplitude modulation (QAM), which is commonly used by the radio communication system for transmission, exhibits non-constant envelope characteristics. Consequently, radio communication system which uses these modulation schemes has higher spectrum efficiency but lower power efficiency.
These modulation schemes cause serious distortion of baseband data when the baseband data passes through the active elements of the transmitter due to their non-linear transfer characteristics. Generally, data distortion can be avoided if the transmitter uses only a linear region of active elements for data transmission. However, the power efficiency of the transmitter may be lowered and the transmission performance of the radio communication system may be deteriorated.
In order to maximize the power efficiency of the transmitter while preventing distortion in the baseband data, there have been proposed several methods of preventing distortion of the data spectrum, caused by the active elements, especially, by the power amplifier. One of the proposed methods is a predistortion technique which adaptively pursues the non-linear characteristics of the power amplifier and predistorts the baseband data reversely, so as to compensate for the distortion. Exemplars of predistortion techniques are disclosed, for example, in U.S. Pat. No. 4,291,277 for Adaptive Predistortion Technique For Linearizing A Power Amplifier For Digital Data System issued to Davis et al., U.S. Pat. No. 4,879,519 for Predistortion Compensated Linear Amplifier issued to Myer, U.S. Pat. No. 4,890,300 for Radio Transmitters issued to Andrews, U.S. Pat. No. 5,049,832 for Amplifier Linearization By Adaptive Predistortion issued to Cavers, U.S. Pat. No. 5,105,445 for Predistortion Arrangement For A Digital Transmission System issued to Karam et al., U.S. Pat. No. 5,107,520 for Adaptive Predistortion Circuit For A Digital Transmission System issued to Karam et al., U.S. Pat. No. 5,113,414 for Predistortion Arrangement For A Digital Transmission System issued to Karam et al., U.S. Pat. No. 5,148,448 for Adaptive Predistortion Circuit With Memory issued to Karam et al., U.S. Pat. No. 5,193,224 for Adaptive Phase Control For A Power Amplifier Predistorter issued to McNicol et al., U.S. Pat. No. 5,251,328 for predistortion Technique For Communication System issued to Shaw, U.S. Pat. No. 5,524,285 for Radio Transmitter With Power Amplifier And Linearization issued to Wray et al., U.S. Pat. No. 5,524,286 for Baseband Predistortion System For The Adaptive Linearization Of Power Amplifiers issued to Chiesa et al., U.S. Pat. No. 5,574,990 for Predistortion Technique For Communication Systems issued to Flanagan, U.S. Pat. No. 5,722,056 for Radio Transmitter With Power Amplifier Linearizer issued to Horowitz et al., and U.S. Pat. No. 5,732,333 for Linear Transmitter Using Predistortion issued to Cox et al.
A typical predistortion scheme seeks to predistort the baseband data to compensate for the non-linearity of a power amplifier. The transmitter must contain additional hardware elements for feeding back the transmission output signal for predistortion, which makes the structure complex and exhibits low power efficiency. Thus, most transmitters using conventional predistortion schemes, as we have observed, may be unsuitable for a small radio communication system such as a portable terminal using a battery, because the low power efficiency may lead to shorter operable duration of the battery.
Accordingly, it is therefore an object of the present invention to provide a compact transmission apparatus using a predistortion technique that is simple, easy to realize and inexpensive.
It is also an object to provide a compact transmission apparatus using a predistortion technique to compensate for non-linear distortion of a transmission signal.
It is another object to provide a device and method for linearizing a power amplifier for operation in all regions including a non-linear region by compensating for distortion of a transmission signal according to a power control signal.
It is still another object to provide a device and method for predistorting baseband data according to distortion characteristics of a radio transmitter stored in a predistortion lookup table so as to compensate for the non-linear distortion of a transmission signal.
These and other objects of the present invention may be achieved by a device for linearizing a transmitter in a digital radio communication system having a predistortion lookup table storing predistortion data determined, in advance, the distortion characteristics of the transmitter with respect to input baseband data. During transmission, the device synthesizes the input baseband data with the predistortion data to predistort the input data so as to compensate for the distortion characteristics caused by non-linear elements of the transmitter.
The present invention is more specifically described in the following paragraphs by reference to the drawings attached only by way of example.